The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. Single-chamber pacemakers generally employ at least one electrode to control the timing of pacing pulses in one chamber of the heart, either an atrium or a ventricle. Normal cardiac excitation begins in the right atrium (RA) and is transmitted to the ventricles through a specialized physiologic conduction system, so an atrial single-chamber pacemaker, which stimulates atrial depolarization, when necessary, best mimics normal physiology. However an atrial single-chamber pacemaker is only effective when the physiologic conduction system is reliable, since ventricular contraction is essential for maintaining hemodynamic perfusion. Ventricular single-chamber pacemakers provide perfusion even if the physiologic conduction system fails, but paced depolarization from a ventricular site may result in less efficient ventricular contraction and thus, inferior hemodynamic perfusion compared to normal, coordinated cardiac excitation.
Dual-chamber pacemakers generally employ atrial and ventricular electrodes, and simultaneously control the timing of pacing pulses to both the RA and one of the ventricles, either right or left. Coordinated pacing of the atrium and ventricle in such a device restores an approximation to normal cardiac contraction. In the nominal case, dual-chamber pacing coordinates atrial and ventricular depolarization, so that each atrial depolarization, whether paced or intrinsic, is followed by a ventricular depolarization, paced, if necessary. More sophisticated dual-chamber pacing algorithms also provide atrial coordination to certain ventricular events, for example inhibiting atrial pacing in response to extraneous ventricular events. However, in recent years, dual-chamber pacing algorithms have been developed to minimize ventricular pacing, for example to reduce energy consumption for increased pacemaker efficiency, while maintaining reasonable coordination between atrial and ventricular contraction.
FIG. 1A illustrates a ventricular single-chamber pacemaker 100, which includes a hermetically sealed canister 101, implanted in a subcutaneous pocket, remote from the heart, and one or more lead wires 106 extending therefrom to corresponding electrodes 111, 112 implanted in a right ventricle RV. Canister 101 may contain a pulse generator 103 like that illustrated via block diagram in FIG. 1B. With reference to FIGS. 1A-B, those skilled in the art will appreciate that pacemaker 100, via electrodes 111, 112, has the capability to sense intrinsic ventricular depolarization (i.e. R-waves) and, in the absence of the intrinsic depolarization, to apply stimulation pulses to the RV in order to create paced ventricular depolarization. Pulse generator 103 of pacemaker 100 further includes rate response sensor 135 that monitors a patient's general level of physical activity to determine an appropriate pacing rate for the patient. Examples of suitable rate response sensors include, without limitation, a force transducing sensor, such as a piezoelectric crystal like that described in commonly assigned U.S. Pat. No. 4,428,378 Anderson et al.; an AC or DC accelerometer like those described in commonly assigned U.S. Pat. No. 5,957,957 to Sheldon; and any type of physiological sensor known in the art, such as those that measure minute ventilation, QT intervals, blood pressure, blood pH, blood temperature, blood oxygen saturation etc. Numerous cardiac pacing methods that employ such RV pacing and sensing and physical activity monitoring are known in the art, for example, as disclosed in commonly assigned U.S. Pat. Nos. 4,428,378 (to Anderson et al.), 6,772,005 (to Casavant et al.), and 5,522,859 (to Stroebel et al.), as well as U.S. Pat. Nos. 5,374,281 (to Kristall et al.) and 6,122,546 (to Sholder et al.). Many of the aforementioned disclosures address the desire to limit the amount of pacing stimulation delivered from implantable pacemakers, particularly right ventricular stimulation in patients that have intact AV conduction (through the AV node, from the sinus node in the right atrial wall to the right and left bundle branches in the ventricular septum), in order to preserve the patient's natural conduction and increase pacemaker efficiency. However, the relatively more sophisticated pacing methods that are geared toward preserving the patient's natural conduction rely upon dual chamber sensing as these methods were developed in concert with the evolution of pacemaker systems from single chamber to dual chamber. Thus, there is a need for new cardiac pacing methods that preserve natural conduction and increase system efficiency for single chamber implantable pacemaker systems, like that shown in FIGS. 1A-B, or like a more compact type of pacemaker which is described in the above-referenced related U.S. patent application Ser. No. 13/192,706.